1. Field of the Invention
The present invention relates to a magnetic recording medium which is employed in, for example, hard disk devices, to a process for producing the magnetic recording medium, and to a magnetic recording and reproducing apparatus. More particularly, the present invention relates to a magnetic recording medium exhibiting excellent recording and reproduction characteristics.
2. Background Art
The recording density of a hard disk device (HDD), which is a magnetic recording and reproducing apparatus, has increased by at least 60% per year, and this tendency is expected to continue. Therefore, magnetic recording heads and magnetic recording media which are suitable for attaining high recording density have been developed.
Most commercially available magnetic recording media for producing hard disk devices are of a longitudinal recording type, in which easy-magnetization axes in a magnetic layer are oriented generally horizontally with respect to a substrate. The term xe2x80x9ceasy-magnetization axisxe2x80x9d refers to an axis which is easily magnetized. In the case of a Co-based alloy, the c axis of a Co hcp structure is an easy-magnetization axis.
When recording density is increased in such a longitudinal magnetic recording medium, the per-bit volume of a magnetic layer becomes excessively small, and recording and reproduction characteristics of the medium may deteriorate because of thermal instability. In addition, when recording density is increased, the effect of a diamagnetic field at a recording bit boundary causes an increase in medium noise.
In contrast, in a perpendicular magnetic recording medium in which easy-magnetization axes in a magnetic film are oriented generally perpendicular to a substrate, even when recording density is increased, a diamagnetic field exerts minimal effect at a recording bit boundary, and recording magnetic domains having clear boundaries are formed, thus enabling noise reduction. Furthermore, even when recording density is increased, bit volume can be relatively increased, and thus thermal stability can be enhanced. Therefore, a perpendicular magnetic recording medium has recently become of interest, and a medium structure suitable for perpendicular magnetic recording has been proposed.
In recent years, there has been a demand for magnetic recording media of higher recording density. In accordance with this trend, employment of a single-pole type head exhibiting excellent ability to record data onto a perpendicular magnetic layer has been proposed. In order to realize employment of such a head, there has been proposed a magnetic recording medium in which a layer formed from a soft magnetic material (called a xe2x80x9csoft back layerxe2x80x9d) is provided between a substrate and a perpendicular magnetic layer serving as a recording layer, to thereby enhance efficiency in magnetic flux flow between the single-pole type head and the medium.
However, the aforementioned magnetic recording medium including a soft back layer is not satisfactory in terms of recording and reproduction characteristics, thermal stability, and recording resolution, and thus demand has arisen for a magnetic recording medium which exhibits excellent recording and reproduction characteristics.
Japanese Patent Application Laid-Open (kokai) No. 2-103715 discloses a magnetic recording medium including a perpendicular magnetic layer formed from a Coxe2x80x94Cr-based alloy to which a rare earth element such as Nd is added as a third element. However, mere addition of Nd to a Coxe2x80x94Cr-based alloy results in insufficient improvement in magnetic characteristics of the resultant magnetic recording medium, including coercive force, the ratio of residual magnetization (Mr) to saturation magnetization (Ms); i.e., Mr/Ms, nucleation field (xe2x88x92Hn), and perpendicular magnetic anisotropy ((Hc-v) (i.e., coercive force in a direction perpendicular to a substrate)/(Hc-i) (i.e., coercive force in a direction parallel to the substrate)). Even when a large amount of Nd is added, satisfactory improvement of the aforementioned magnetic characteristics attributed to an increase in magnetic anisotropy constant (Ku) was not attained, and recording and reproduction characteristics may be deteriorated as a result of an increase in noise.
As a general measure, there has been proposed a technique for incorporating a large amount of Pt (16 to 26 at %) into a Co alloy employed for forming a magnetic layer in order to increase the magnetic anisotropy constant (Ku) of the Co alloy. However, when a magnetic layer is formed from a Co alloy material containing a large amount of Pt, magnetic interaction between grains in the magnetic layer becomes very strong, and thus noise becomes large, thereby rendering the magnetic layer unsuitable for high-density recording.
Therefore, demand has arisen for a magnetic material which can suppress an increase in noise while increasing Ku in a perpendicular direction, and a magnetic recording medium exhibiting high Ku and low noise.
In view of the foregoing, an object of the present invention is to provide a magnetic recording medium which enables improvement of recording and reproduction characteristics and thermal stability, to thereby attain high-density recording and reproduction of data.
Another object of the present invention is to provide a process for producing the magnetic recording medium.
Yet another object of the present invention is to provide a magnetic recording and reproducing apparatus including the medium.
In order to solve the aforementioned problems, the present inventors have performed extensive studies on a technique for incorporating Nd into a magnetic material containing Co as a primary component and also containing Cr and Pt, and on the relation between compositional proportions of these elements and recording and reproduction characteristics of a magnetic recording medium, to thereby accomplish the present invention. Accordingly, the present invention provides the following embodiments for solving the aforementioned problems.
1) A magnetic recording medium comprising, in order, a non-magnetic substrate; an orientation-regulating layer for regulating the crystal orientation of a layer provided directly thereon; a perpendicular magnetic layer in which easy-magnetization axes are oriented generally perpendicular to the substrate; and a protective layer; and wherein the perpendicular magnetic layer is formed from a material containing Co as a primary component and at least Cr, Pt, and Nd, a ratio of residual magnetization (Mr) in a direction perpendicular to the substrate to saturation magnetization (Ms) in a direction perpendicular to the substrate s at least 0.85, and an activation magnetic moment represented by the product of activation volume and saturation magnetic moment is 0.3xc3x9710xe2x88x9215 emu to 0.8xc3x9710xe2x88x9215 emu.
2) A magnetic recording medium according to 1), wherein a coercive force in a direction perpendicular to the substrate is at least 2,500 Oe, and a ratio of the coercive force (Hc-v) in a direction perpendicular to the substrate to the coercive force (Hc-i) in a direction parallel to the substrate is at least 5.
3) A magnetic recording medium according to 1) or 2), wherein a nucleation field in a direction perpendicular to the substrate is 0 to 2,000 Oe.
4) A magnetic recording medium according to 1) or 2), wherein the perpendicular magnetic layer is formed from a material containing Cr in an amount of 18 to 28 at %, Pt in an amount of 10 to 20 at %, and Nd in an amount of 0.5 to 8 at %.
5) A magnetic recording medium according to 1) or 2), wherein the perpendicular magnetic layer is formed from a material further containing at least one element selected from the group consisting of B, Ta, and Cu, in which the total amount of these elements is 8 at % or less.
6) A magnetic recording medium comprising a perpendicular magnetic layer containing a plurality of magnetic layers, wherein at least one of the magnetic layers is a perpendicular magnetic layer as recited in 1) or 2).
7) A magnetic recording medium comprising a perpendicular magnetic layer containing a plurality of magnetic layers, wherein the uppermost layer of the magnetic layers is a perpendicular magnetic layer as recited in 1) or 2).
8) A magnetic recording medium according to 1) or 2), wherein a soft magnetic undercoat layer formed from a soft magnetic material is provided between the non-magnetic substrate and the orientation-regulating layer.
9) A magnetic recording medium according to 8), wherein the surface of the soft magnetic layer is oxidized or nitridized.
10) A magnetic recording medium according to 8) or 9), wherein a hard magnetic layer in which magnetic anisotropy is generally in a longitudinal direction is provided between the non-magnetic substrate and the soft magnetic undercoat layer.
11) An magnetic recording medium according to 10), wherein the hard magnetic layer is formed from a material containing a CoSm alloy or a CoCrPtX2 alloy (wherein X2 is at least one element selected from the group consisting of Pt, Ta, Zr, Nb, Cu, Re, Ni, Mn, Ge, Si, O, N, and B, has a coercive force of at least 500 Oe, and has a magnetization direction along a radial direction of the substrate.
12) A process for producing a magnetic recording medium, which comprises successively forming on a non-magnetic substrate, an orientation-regulating layer for regulating the crystal orientation of a layer provided directly thereon, a perpendicular magnetic layer in which easy-magnetization axes are oriented generally perpendicular to the substrate, and a protective layer, wherein,
the perpendicular magnetic layer is formed through sputtering by use of a sputtering target comprising a material containing Co as a primary component and at least Cr, Pt, and Nd, and wherein the Cr content is 18 to 28 at %, the Pt content is 10 to 20 at %, and the Nd content is 0.5 to 8 at %.
13) A process for producing a magnetic recording medium according to 12), wherein, when the perpendicular magnetic layer is formed, the pressure of a sputtering gas is regulated to 3 to 20 Pa.
14) A process for producing a magnetic recording medium according to 12) or 13), which further comprises forming a soft magnetic layer before formation of the orientation-regulating layer, and exposing the surface of the soft magnetic layer to a gas selected from among oxygen gas, nitrogen gas, a mixture of oxygen gas and nitrogen gas, and a gas containing oxygen gas and nitrogen gas.
15) A magnetic recording and reproducing apparatus comprising a magnetic recording medium and a magnetic head for recording data onto the medium and reproducing the data therefrom, wherein the magnetic recording medium is a magnetic recording medium as recited in 1) or 2).
As used herein, the term xe2x80x9cprimary componentxe2x80x9d refers to a component which is contained in an alloy in the greatest amount, preferably in an amount of more than 50 at %.